The invention is generally related to monolithic coupled-dual resonator crystals and in particular is directed to a four frequency measuring process and apparatus for accurately measuring the characteristics of such crystals having relatively high frequency fundamental and overtone modes and/or relatively high resonator resistances. The exemplary embodiments disclosed are directed not only toward the accurate measuring of crystal parameters at any step of the crystal fabrication process after the resonators have been formed, but are also directed toward permitting the manufacture of precision coupled-dual resonator crystals that are operable at frequencies well above those presently available.
The general overall structure and theory of operation as well as methods for making such monolithic structures are well known as indicated in some commonly assigned generally related prior patents listed below.
U.S. Pat. No. 3,992,760--Roberts (1976)
U.S. Pat. No. 4,093,914--Peppiatt et al (1978)
U.S. Pat. No. 4,627,379--Roberts et al (1986)
U.S. Pat. No. 4,676,993--Roberts et al (1987)
U.S. Pat. No. 4,839,618--Roberts et al (1989)
A typical dual resonator crystal includes a plate of piezoelectric material having an input electrode and an output electrode on one face of the plate and a common electrode on the opposite face of the plate. A first resonator is formed by the input electrode and common electrode, and the second resonator is formed by the output electrode and the common electrode. Such dual resonator crystal filters are used extensively in electronic applications where devices having very narrow bandpass filter characteristics are desired. Although such devices are mass produced using automated techniques, control of parameters such as electrode locations, thicknesses and spacings are critical, particularly where high center frequencies are required. Such manufacturing techniques require accurate real time measurements of key coupled-dual crystal characteristics during the fabrication process and in particular in the final frequency adjust process.
As indicated in commonly assigned U.S. Pat. No. 4,093,914, accurate determination of desired characteristics such as the resonant frequency of each of the dual resonators (FA and FB) as well as the normalized center frequency of the device are desirable. Moreover, determination of "synchronous peak separation frequency" (SPSF) is indicated to be of particular significance since it provides a common reference value at a particular point in a process of manufacturing a specific crystal design such that each crystal in the group may be properly evaluated by being mathematically related to standard conditions. The key crystal parameters, including synchronous peak separation frequency, are calculated based upon four critical frequencies determined by either of two disclosed methods. Both of the disclosed methods involve obtaining the four critical frequencies by monitoring one of the two crystal ports while either shorting or effectively open circuiting the second port. The methodology taught in this commonly assigned patent to Peppiatt and Roberts is the only process known to us for accurately obtaining the critical frequencies necessary for the calculation of the key crystal parameters, such as resonator frequencies and synchronous peak separation frequency, at any step in the fabrication process after the electrodes have been formed as well as at the final test stage.
At the time that the Peppiatt and Roberts methodology was discovered, coupled-dual crystals at 21.4 MHz were the highest frequency units commonly made. At the present time, however, units at 45 MHz and 57.5 MHz are currently in production. Moreover, because of the requirements for higher and higher IF filters for cellular radios and the like with 70 MHz and 90 MHz units being used or considered in new product applications, the continued availability of an accurate measuring process for such high frequency coupled-dual resonator crystals is vitally important for measuring crystal characteristics and for producing precision units.
Briefly stated, we have discovered that as the desired fundamental or overtone operating frequencies of such crystal structures increase and/or the effective resonator resistances increase, the measured phase excursions below the zero reference diminish and eventually fail to cross the zero reference. We have additionally discovered that where one of the resonator frequencies (FA, for example) is much lower than that of the other resonator frequency (FB), two of the measured frequencies (F1 and F2) will be markedly higher in amplitude than the other two and the latter may not exhibit excursions below the zero reference.
Since such zero crossings are necessary for accurate frequency measurements, compensating circuitry is required in order to establish these reference points in the Peppiatt and Roberts transmission measurement system when applied to high frequency and/or high resonator resistance crystal filters to thereby produce accurate and precise measured results. Such compensation not only solves the problem of obtaining zero phase crossing points under the noted conditions, but also solves the basic problem of how to accurately measure the parameters of coupled-dual resonator crystals at any step during the manufacturing process following the formation of the resonators where the selected crystal design previously would have prevented obtaining such measurements.